Such printing presses are known as such in the art, in particular from European Patent Publication No. EP 0 949 069 A1 and International PCT Publications Nos. WO 2007/042919 A2, WO 2007/105059 A1, WO 2007/105061 A1 and WO 2008/099330 A2, which publications are all incorporated herein by reference in their entirety.
International PCT Publication No. WO 2007/042919 A2 in particular discloses such a recto-verso printing press that further comprises an additional printing group placed upstream of a main printing group of the printing press.
FIGS. 1 and 2 illustrate such a recto-verso printing press that is adapted to carry out simultaneous recto-verso printing of sheets, as typically used for the production of banknotes and like security documents, which printing press is designated globally by reference numeral 100. Such printing press is in particular marketed by the present Applicant under the product designation Super Simultan® IV. The basic configuration of the printing press 100 shown in FIGS. 1 and 2 is similar to that shown and discussed with reference to FIG. 1 of International PCT Publication No. WO 2007/042919 A2.
This printing press 100 comprises a printing unit 2, which is specifically adapted to perform simultaneous recto-verso printing of the sheets (according to the so-called Simultan-offset printing principle) and comprises, as is typical in the art, two blanket cylinders (or printing cylinders) 5, 6 rotating in the direction indicated by the arrows and between which the sheets are fed to receive multicolour impressions. In this example, blanket cylinders 5, 6 are three-segment cylinders which are supported between a pair of side frames designated by reference numeral 20. The blanket cylinders 5, 6 receive and collect different ink patterns in their respective colours from plate cylinders 15 and 16 (four on each side) which are distributed around a portion of the circumference of the blanket cylinders 5, 6. These plate cylinders 15 and 16, which each carry a corresponding printing plate, are themselves inked by corresponding inking apparatuses 25 and 26, respectively. The two groups of inking apparatuses 25, 26 are advantageously supported in two inking carriages 21, 22 that can be moved toward or away from the centrally-located plate cylinders 15, 16 and blanket cylinders 5, 6.
As is known in the art, each printing plate is wrapped around the corresponding plate cylinder 15, 16 and clamped at its leading end and trailing end by a suitable plate clamping system, which plate clamping system is located in a corresponding cylinder pit of the plate cylinder (see e.g. International (PCT) Publications Nos. WO 2013/001518 A2, WO 2013/001009 A1 and WO 2013/001010 A2, which are also incorporated herein by reference in their entirety).
Sheets are fed from a sheet feeder 1 onto a feeder table located next to the printing unit 2 (on the right-hand side in FIGS. 1 and 2) to a succession of transfer cylinders 9, 8′, 10 (three cylinders in this example) placed upstream of the blanket cylinders 5, 6. While being transported by the transfer cylinder 8′, the sheets receive a first impression on one side of the sheets using an additional printing group, the transfer cylinder 8′ fulfilling the additional function of impression cylinder. This additional printing group consists of, in addition to the transfer cylinder 8′, a blanket cylinder 8 (a two-segment cylinder in this example) that collects inks from two plate cylinders 18 that are inked by corresponding inking apparatuses 28. The inking apparatuses 28 are advantageously supported in an inking carriage 24 that can be moved toward or away from the plate cylinders 18 and blanket cylinder 8. The sheets that are printed by means of the additional printing group are first dried/cured by a drying/curing unit (not referenced in FIGS. 1 and 2) while being transported by the sheet transfer cylinder 8′ before being transferred to the downstream-located main printing group.
In the example of FIGS. 1 and 2, the sheets are transferred onto the surface of blanket cylinder 5 where a leading edge of each sheet is held by appropriate gripper means located in cylinder pits between each segment of the blanket cylinder 5. Each sheet is thus transported by the blanket cylinder 5 to the printing nip between the blanket cylinders 5 and 6 where simultaneous recto-verso printing occurs. Once printed on both sides, the printed sheets are then transferred, as known in the art, to a sheet conveying system 3 (such as a chain gripper system with spaced-apart gripper bars) for delivery in a sheet delivery unit 4 comprising multiple (e.g. three) delivery pile units. Reference numeral 31 in FIG. 2 designates a pair of chain wheels located at the upstream end of the sheet conveying system 3.
In the example of FIGS. 1 and 2, first and second transfer cylinders or drums 11, 12, such as suction drums or cylinders, are interposed between the sheet conveying system 3 and the blanket cylinder 5. These first and second transfer cylinders 11, 12 are optional and designed to carry out inspection of the sheets on the recto and verso sides as described for instance in International application No. WO 2007/105059 A1. Reference numerals 61, 62 in FIG. 2 designate corresponding inspection cameras (such as line-scan cameras) that cooperate with cylinder or drums 11, 12.
A limitation of the known solutions as described above resides in the fact that the additional printing group is only capable of printing one side of the sheets.
There is therefore a need for an improved solution, especially such a solution that expands the printing capabilities of the printing press